In recent years, fuel cells have been garnering attention as high efficiency energy conversion devices. Fuel cells are roughly classified by the type of electrolyte used into alkali types, solid polymer types, phosphoric acid types, and other low temperature operation fuel cell, molten carbonate types, solid oxide types, and other high temperature operation fuel cells. Among these, solid polymer type fuel cells (PEFC) using polymer electrolyte membranes having ion conductivity as electrolytes enable a high output density to be obtained by a compact structure and further do not use a liquid for the electrolyte and enable operation at a low temperature etc., so enable realization of a simple system, so are being focused on for stationary use, vehicular use, mobile phone use, etc.
A solid polymer type fuel cell has as its basic principle to expose one side of a polymer electrolyte membrane to a fuel gas (hydrogen etc.) and its opposite side to an oxidizing agent gas (air etc.), use the chemical reaction via the polymer electrolyte membrane to synthesize water, and take out the reaction energy produced due to this as electricity. A polymer electrolyte membrane at the two sides of which porous catalyst electrodes are arranged and joined by a hot press etc. is generally called a “membrane electrode assembly (MEA)”. An MEA can be independently handled. Packing is arranged between the MEA and separators so as to prevent leakage of the reaction gases to the outside. A polymer electrolyte membrane has ion conductivity, but does not have gas permeability and electron conductivity, so acts to physically and electronically separate the fuel electrode and the oxygen electrode. If the polymer electrolyte membrane is smaller in size than the porous catalyst electrodes, at the inside of the MEA, the porous catalyst electrodes will electrically short-circuit and, further, the oxidizing agent gas and fuel gas will mix (cross leak), so the function as a battery will be lost. Furthermore, in the case of a type of fuel cell which directly feeds methanol or another liquid fuel, the liquid fuel will leak from the fuel electrode side to the oxygen electrode side and thereby the function as a battery will be impaired. For this reason, the area of the polymer electrolyte membrane has to be equal to or greater than the areas of the porous catalyst electrodes. Therefore, usually, the polymer electrolyte membrane is made to extend beyond the peripheral edges of the porous catalyst electrodes and is clamped between the packing and separators so as to form a gas seal and supporting structure.
In this respect, a polymer electrolyte membrane is an extremely thin film shaped material, so is hard to handle. When bonded with the electrodes, at the time of assembly stacking a plurality of unit cells to form a stack, etc., the peripheral edges, which are important for sealing the reaction gases, frequently end up being wrinkled. In a unit cell or stack assembled using a polymer electrolyte membrane in such a wrinkled state, there is a high possibility of the reaction gases leaking out from the wrinkled parts. Further, even in a state with no wrinkles etc. at all, the polymer electrolyte membrane is the member with the lowest mechanical strength among all constituent members forming a stack, so is easily damaged. Therefore, to improve solid polymer type fuel cells in reliability, maintenance, etc., reinforcement of the polymer electrolyte membrane part is desired. Furthermore, as explained above, to prevent electrical short-circuits at the peripheral edges of the polymer electrolyte membrane, in the past, MEAs including electrolyte membranes with areas larger than the electrode layers so that the polymer electrolyte membranes extend laterally over the ends of the electrode layers have been produced. However, when fabricating an MEA with an electrolyte membrane and electrode layers of different sizes, these have to be separately cut out and positioned. The increase in processes invites a drop in productivity.
Known in the art is a method of applying to the peripheral edges of an MEA, which has a polymer electrolyte membrane of the same size as the gas diffusion electrodes or larger than the gas diffusion electrodes, a thermoplastic polymer by injection molding, compression molding, or other means so that the thermoplastic polymer is impregnated at the insides of the sealing ends of the gas diffusion support members and envelops the peripheral regions of both gas diffusion support members and the polymer electrolyte membrane thereby forming an integral membrane electrode assembly having a fluid impermeable seal of a thermoplastic polymer (PLT 1).
Further, to effectively reinforce the polymer electrolyte membrane and greatly improve the handling ability of a fuel cell structure, the method is known of press fitting a frame member over the outer peripheral edges of the porous bodies fastened to the two surfaces of the polymer electrolyte membrane and strongly and reliably joining the porous bodies and frame member (PLT 2).